Space management for a hierarchical set of file systems

A computer program product for storage management in a hierarchical set of file systems, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to:

• create, by the processor, a structure for the hierarchical set of file systems including a top-file system and sub-file systems that each include files and directories;
• track, by the processor, directories for the sub-file systems of the hierarchical set of file systems;
• maintain, by the processor, a directory while providing the directory is capable of residing in more than one sub-file system;
• for each directory in the hierarchical set of file systems:
• maintain, by the processor, a corresponding directory in the top-file system;
• and
• maintain, by the processor, a number of symbolic pointers in the corresponding directory which each point from the corresponding directory to a sub-file system where any portion of the tracked directory resides;
• and
• upon a request by a particular sub-file system for more inodes without any inodes available, revoke, by the processor, inode numbers from at least one other sub-file system having unused inodes;
• wherein per-dataset failure isolation is limited by placement of files across multiple sub-file systems, and each directory in the sub-file systems is named using an inode number of a directory in the top file system.

An apparatus comprising:

• a memory storing instructions;
• and
• one or more processors executing the instructions to:
• create a structure for a hierarchical set of file systems including a top-file system and sub-file systems that each include files and directories;
• track directories for the sub-file systems of the hierarchical set of file systems;
• maintain a directory while providing the directory is capable of residing in more than one sub-file system;
• and
• upon a request by a particular sub-file system for more inodes without any inodes available, revoke inode numbers from at least one other sub-file system having unused inodes;
• wherein per-dataset failure isolation is limited by placement of files across multiple sub-file systems, and each directory in the sub-file systems is named using an inode number of a directory in the top file system.